Inductively coupled plasma mass spectrometry (ICPMS) provides accurate quantitative information on major, minor, trace, and ultra-trace elements of industrial, geological, environmental, and biological samples. In ICPMS, an aerosol sample is carried by a carrier gas stream to a so-called ICP torch. In this torch, the gas is subjected to intense high-frequency electromagnetic fields, which lead to the formation of a plasma by induction. The ions from the plasma are then extracted into a mass spectrometer, where they are separated on the basis of their mass-to-charge ratios.
ICPMS can be coupled with laser ablation (LA) to ablate material from a solid sample so as to create the aerosol required for ICP. Ablation may be carried out directly in the ICP torch, or the sample may be placed in an external laser ablation cell upstream of the ICP torch, and the aerosol created by laser ablation is transported to the ICP torch by the carrier gas stream. For example, reference 1 demonstrated a laser ablation cell (the so-called HEAD cell) for which the aerosol ejection direction is parallel to that of the carrier gas. Another laser ablation cell design based on a similar principle is demonstrated in reference 2.
Since the first half of 1990s, attempts have been made to use laser-ablation ICPMS (LA-ICPMS) as a chemical imaging tool by scanning the laser spot over the sample surface. Many studies have demonstrated the potential imaging capabilities of LA-ICPMS based on a considerable variety of hard and soft samples. Most of these studies showed an effective spatial resolution of approximately 5-100 μm. Although LA-ICP-MS offers highly multiplexed quantitative analysis of antigen expression in single cells, it currently lacks the resolution necessary for the imaging of single cells within tissue samples.
However, some applications, such as diagnostic analysis of tissue sections, requires higher spatial resolution, e.g. to visualize cell-to-cell variability. The effective spatial resolution is determined by the laser spot size convoluted with the system dispersion. The system dispersion is in turn often dominated by a compromise between the aerosol washout time after each laser shot and the scanning speed. The longer the washout time, the more overlap will occur between signals originating from neighboring sample spots if the scanning speed is kept fixed. Therefore, aerosol washout time often is one of the key limiting factors for improving resolution without increasing total scan time.
The fastest washout time can be achieved by in-torch ablation, resulting in single shot signal durations of a few milliseconds. However, in-torch ablation is limited to very small samples, and scanning of the laser spot is very difficult to realize with in-torch ablation. Therefore, for imaging applications, external laser ablation cells are generally employed. However, even with the best known cell designs, washout times are often on the scale of seconds, and short washout times under 100 milliseconds are hard to achieve.
It is an object of the invention to provide further and improved laser ablation cells, ablation apparatus incorporating such cells (for example, linked to an ICP-MS), which have an application in techniques for imaging of biological material, such as tissue samples, monolayers of cells and biofilms, and in particular to adapt LA-ICP-MS for use as a single-cell imaging technique.